The present invention relates to symbol timing, and more particularly to symbol timing maintenance in a communications receiver. Even more particularly, the present invention relates to symbol timing maintenance in a receiver to enable low duty cycle receiver operation.
In mobile communications systems, mobile units, or subscriber units, are typically powered by battery power sources, such as Nickel cadmium, Nickel Metal Hydride, Lithium, Lead Acid, Carbon Zinc, or Alkaline Batteries. These battery power sources provide a limited supply of power to the subscriber unit before being depleted, and then need to be recharged or replaced. As a result, this limited supply of power should be utilized in as highly efficient of a manner as possible, minimizing times and amounts of power consumption not essential to operation of the subscriber unit, while still allowing performance of tasks necessary to subscriber unit operation.
When the subscriber unit is not "in use", i.e., is not actively being used by a user to communicate, the subscriber unit generally monitors a channel for "pages". These "pages" may indicate to the subscriber unit that there is an incoming call (in which case the pages can be referred to as call initiation messages), and can be used by the subscriber unit to initiate, for example, the ringing of a ringer. Alternatively, these "pages" may indicate to the subscriber unit the receipt of a page, in which case the subscriber unit initiates a beeping tone or vibrates, in order to signal to a user that he or she has been paged.
In order to maximize power usage efficiency within the subscriber unit, the subscriber unit is preferably put into a "stand by" mode, during which transceiver and processing sections of the phone are shut down, except for a clock that is used to "wake up" the subscriber unit periodically, whenever possible. For example, the subscriber unit may be put in "stand by" mode between periodic time slots during which the subscriber unit must monitor the channel for "pages." Prior to such periodic time slots, the subscriber unit is "awakened." Such "waking up" of the subscriber unit is necessary so that the subscriber unit can "listen" to the control channel for incoming page signals during the periodic time slots. In the event that the system in which the subscriber unit operates is required to periodically repeat "pages," it may even be possible and desirable to "wake up" the subscriber unit only during selected ones of the periodic time slots, such as during every other one, or every third one of the periodic time slots. While this may result in some delay in receiving incoming pages, for, for example, three to ten seconds, the concomitant power savings may, under some circumstances, easily justify this trade off.
Thus, in battery-powered wireless communications systems, it is highly desirable that "stand-by" time be maximized in mobile or subscriber units and that "awake" or active mode time be minimized. In order to maximize standby time, the subscriber unit must spend as little time as possible in the active mode, while still periodically monitoring "pages," e.g., call initiation messages.
To this end, the protocol used to transmit the "pages" should transmit such "pages" during determinable periods spaced apart in time as far as possible, and as short as possible in length. These periods may be spaced in time at regular intervals or their spacing may be more complex.
The subscriber unit preferably "wakes up" as infrequently as possible while still allowing it to receive the "pages" at a reasonable interval, in accordance with the subscriber unit's particular application. The periods of time during which the subscriber unit remains in the active state should be minimized, i.e., activities such as power-up-style reacquisition of accurate signal timing should be avoided.
Problematically, however, factors such as imprecision in internal clock rate within the subscriber unit, imprecision in clock rate in a base station, drift in clock rate due, for example, to temperature, and the like cause inaccuracies in timing between the base station and the subscriber unit over time. These inaccuracies limit the amount of time during which the subscriber unit can "standby" and the shortness of periods during which the subscriber unit remains active. These inaccuracies have also required heretofore that the subscriber unit periodically perform power-up-style timing reacquisition in order to reacquire accurate timing synchronization with, for example, a base station. Unfortunately, both the limitation on the time during which the subscriber unit can "standby" and the need for periodic reacquisition, result in higher power consumption and shorter battery life than would otherwise be attainable.
The present invention advantageously addresses the above and other needs.